In recent years high-speed transmission has been achieved using an optical transmission system. A 10 Gbps optical transmission system has become in practical use. Currently 40 Gbps optical transmission system is under development. Meanwhile, as transmission speed increases, optical signal waveform becomes deteriorated caused by wavelength dispersion in an optical fiber, a polarized wave dispersion in both the optical fiber and an optical circuit, nonlinear effect, ASE (amplified spontaneous emission) noise produced in an optical amplifier, etc. This deterioration of optical signal waveform becomes a main factor to restrict transmission distance.
For this reason, when transmitting for more than hundreds of kilometers with the speed of 40 Gbps, compensation against waveform deterioration is strictly required. When compensating, however, a plurality of automatic compensation systems are required to cope with the above-mentioned various factors producing waveform deterioration.
Further, to control such compensation, it is required to minimize the deterioration of transmission quality caused by this control itself.
One method for controlling transmission characteristic using a plurality of compensators, there is disclosed a configuration in the official gazette of Japanese Unexamined Patent Publication No. Hei-9-326755, which is shown in FIG. 1. The system is configured with a receiver 100 and an automatic equalization controller 200. In receiver 100, an optical amplifier 101 is provided for optical-amplifying the signal with excited light produced by noise light generator 201 in automatic equalization controller 200.
The amplified optical reception signal is dispersively compensated in a variable dispersion compensation circuit 102 to convert into electric signal in opt-electric converter 103. An electric signal corresponding to the converted reception signal is amplified in an amplifier 104 to input to a clock extraction circuit 105 and a discrimination circuit 106. In discrimination circuit 106, electric signal level is discriminated at the timing of a clock signal regenerated in clock extraction circuit 105 according to the discrimination signal from an discrimination voltage generation circuit 107, to output to a reception code.
Meanwhile, in automatic equalization controller 200, an output error of discrimination circuit 106 is detected in an error detection circuit 202. Corresponding to the output from error detection circuit 202, noise light level in noise light generator 201 is controlled. Also, through a dispersion equalization controller 203 and a discrimination voltage controller 204, feedback control is performed respectively against variable dispersion compensation circuit 102 and discrimination voltage generation circuit 107.
In the conventional method shown in FIG. 1, on detecting an output error in error detection circuit 202, the feedback control is carried out by a variety of compensation circuits. Namely, in case of the configuration shown in FIG. 1, such compensation circuits include noise light generator 201, variable dispersion compensation circuit 102, discrimination voltage generation circuit 107.
As a variable wavelength dispersion compensation device constituting variable dispersion compensation circuit 102, there are employed a VIPA (virtually imaged phased array) device such as being disclosed by M. Shirasaki, et al., in “Dispersion compensation using the virtually imaged phased array” (APCC/OECC '99, pp. 1367–1370), and an FBG (fiber Bragg grating) such as being disclosed by M. M. Ohn, et al. in “Tunable fiber grating dispersion using a piezoelectric stack” (OFC '97, WJ3).
In the configuration shown in FIG. 1, a device such as VIPA (virtually imaged phased array) is applied as a variable wavelength-dispersion compensation device for dispersion compensation to vary dispersion values. In order to search a setting point to optimize a dispersion value, it is not possible to determine to which direction the search is carried out optimally (that is, the direction of the setting value to be varied either greater or smaller). For this reason, conventionally the control direction to the optimal setting point is determined by searching in any directions possible to search, thereby to monitor how many amount the transmission deterioration is produced.
In this method, if the selected moving direction is opposite to the direction of optimal setting, the transmission deterioration increases by the search. In addition, in the conventional method, a threshold value of the transmission deterioration is set so as to suspend to start until the deterioration exceeds the threshold value. Therefore, substantially large transmission deterioration is produced before the control is activated.
Also, in the conventional plurality of compensation methods against the control device, there is introduced a centralized control method in which each compensation circuit is managed with centralized control. In such a centralized control method, the control management becomes difficult as the number of compensation circuits increases. At the same time, it becomes difficult to increase the number of compensation circuits corresponding to the increase of communication speed or the expansion of communication distance. Therefore the method is short of expandability and flexibility.